Grimoire cards for some guns were available as codes, at the start of the game, so not an accurate track
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They can see how many kills you have with the exotic, not 100% foolproof but it's hard to believe you wouldn't take an exotic for a test run before trashing it or storing it in your vault.
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But there is a specific grimoire card for each exotic under the inventory cards.
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I think those grimoire codes didn't unlock the grimoire card you unlock when you actually obtain the gun